Semiconductor wafer thinning systems and related methods

ABSTRACT

Semiconductor substrate thinning systems and methods. Implementations of a method of thinning a semiconductor substrate may include: providing a semiconductor substrate having a first surface and a second surface opposing the first surface and inducing damage into a portion of the semiconductor substrate adjacent to the second surface forming a damage layer. The method may also include backgrinding the second surface of the semiconductor substrate.

BACKGROUND 1. Technical Field

Aspects of this document relate generally to systems and methods forthinning wafers, such as systems and methods for thinning semiconductorsubstrates. More specific implementations involve systems and methodsfor thinning silicon carbide substrates.

2. Background

Semiconductor substrates are typically thinned following separation froma boule using a backgrinding process. The backgrinding process usuallyinvolves grinding a surface of the substrate using a grinding wheelwhich includes a plurality of teeth and rotates over the surface of thesubstrate.

SUMMARY

Implementations of a method of thinning a semiconductor substrate mayinclude: providing a semiconductor substrate having a first surface anda second surface opposing the first surface and inducing damage into aportion of the semiconductor substrate adjacent to the second surfaceforming a damage layer. The method may also include backgrinding thesecond surface of the semiconductor substrate.

Implementations of the method of thinning may include one, all, or anyof the following:

A lifetime of a backgrinding wheel may be increased through the presenceof the damage layer.

After backgrinding the second surface of the semiconductor substrate,the method may include inducing damage into a portion of thesemiconductor substrate adjacent to the second surface forming a seconddamage layer and backgrinding the second surface of the semiconductorsubstrate to remove at least the second damage layer.

After inducing damage into a portion of the semiconductor substrate, themethod may include again inducing damage into a portion of thesemiconductor substrate adjacent to the second surface beforebackgrinding the second surface of the semiconductor substrate.

A thinning rate may be increased while a backgrinding wheel grinds thedamage layer.

Inducing damage into the portion of the semiconductor substrate mayfurther include irradiating the second surface with a laser beam at afocal point within the semiconductor substrate at a plurality of spacedapart locations along the second surface to form the damage layer.

The method may further include forming one or more cracks into thesemiconductor substrate surrounding each of the plurality of spacedapart locations.

The semiconductor may be silicon carbide.

Irradiating the second surface with the laser beam at the focal pointwithin the semiconductor substrate at the plurality of spaced apartlocations may further include irradiating the plurality of spaced apartlocations using a predefined path.

In various method implementations, the predefined path may be analternating single pass path, an intersecting single pass path, a spiralsingle pass path, an alternating dual pass path, a dual intersectingpass path, a spiral dual pass path, a random single pass path, a randomdual pass path, a single pass path, a two or more pass path, anintersecting single pass path, an intersecting dual pass path, anoverlapping single pass path, an overlapping dual pass path, or anycombination thereof.

Inducing damage into the portion of the semiconductor substrate mayfurther include bombarding the second surface with a plurality of ionsfrom a plasma adjacent to the second surface to form the damage layer.

Inducing damage into the portion of the semiconductor substrate mayfurther include implanting the second surface with a plurality of ionsto form the damage layer.

Inducing damage into the portion of the semiconductor substrate mayfurther include exposing the second surface to an etchant to form thedamage layer.

Inducing damage into the portion of the semiconductor substrate mayfurther include locally rapidly cooling the second surface to form thedamage layer.

Inducing damage into the portion of the semiconductor substrate mayfurther include locally rapidly heating the second surface to form thedamage layer.

The method may include locally rapidly heating the first surface whilelocally rapidly cooling the second surface to form the damage layer.

The method may include locally rapidly heating the second surface whilelocally rapidly cooling the first surface to form the damage layer.

Implementations of a method of preparing a semiconductor substrate forthinning may include providing a semiconductor substrate having a firstsurface and a second surface opposing the first surface. The method mayinclude forming a damage layer in a portion of the semiconductorsubstrate adjacent to the second surface where the damage layer isconfigured to increase a thinning rate when a backgrinding wheel isgrinding the damage layer.

Implementations of the method of preparing a semiconductor substrate forthinning may include one, all, or any of the following:

Forming the damage layer in the portion of the semiconductor substrateadjacent to the second surface may further include irradiating thesecond surface with a laser beam at a focal point within thesemiconductor substrate at a plurality of spaced apart locations alongthe second surface to form the damage layer.

The substrate may be silicon carbide.

The method may further include forming one or more cracks into thesemiconductor substrate surrounding each of the plurality of spacedapart locations.

Forming the damage layer in the portion of the semiconductor substrateadjacent to the second surface may further include bombarding the secondsurface with a plurality of ions to form the damage layer, implantingthe second surface with a plurality of ions to form the damage layer,exposing the second surface to an etchant to form the damage layer,locally rapidly cooling the second surface to form the damage layer,locally rapidly heating the second surface to form the damage layer,locally rapidly heating the first surface while locally rapidly coolingthe second surface to form the damage layer, locally rapidly heating thesecond surface while locally rapidly cooling the first surface to formthe damage layer, or any combination thereof.

The foregoing and other aspects, features, and advantages will beapparent to those artisans of ordinary skill in the art from theDESCRIPTION and DRAWINGS, and from the CLAIMS.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations will hereinafter be described in conjunction with theappended drawings, where like designations denote like elements, and:

FIG. 1 is a cross sectional view of a semiconductor substrate (siliconcarbide in this case) being irradiated using a laser forming a pluralityof spaced apart locations;

FIG. 2 is a cross sectional view of another semiconductor substrate witha finished device layer on a first surface being irradiated using alaser on a second surface;

FIG. 3 is a cross sectional view of a silicon carbide semiconductorsubstrate (silicon carbide substrate) being irradiated using a laser ona second surface after the second surface has been separated from aboule;

FIG. 4 is a cross sectional view of another silicon carbide substratebeing irradiated using a laser on a second surface, the second surfaceof the silicon carbide substrate having a laser dispersion reductionlayer thereon;

FIG. 5 is a cross sectional view of another silicon carbide substratebeing irradiated by a laser on a first side following irradiation of thesecond side of the substrate;

FIG. 6 is a cross sectional view of another silicon carbide substratebeing irradiated by a laser on a first side and being irradiated by alaser on a second side simultaneously on opposite portions of thesubstrate;

FIG. 7 is a cross sectional view of another silicon carbide substratebeing irradiated by a laser on a first side and being irradiated by alaser on a second side simultaneously on positionally aligned portionsof the substrate;

FIG. 8 is a cross sectional view of another silicon carbide substratebeing irradiated by two laser beams on a second side of the substrate,the two laser beams being spaced apart across a midpoint of thesubstrate;

FIG. 9 is a cross sectional view of another silicon carbide substratebeing irradiated by two laser beams spaced closely together;

FIG. 10 is a diagram of a semiconductor substrate with an alternatingsingle pass laser irradiation path (single pass path) illustratedthereon;

FIG. 11 is a diagram of a semiconductor substrate with an intersectingdual pass path illustrated thereon;

FIG. 12 is a diagram of a semiconductor substrate with a spiral singlepass path illustrated thereon;

FIG. 13 is a diagram of a semiconductor substrate with an angledintersecting dual pass path illustrated thereon;

FIG. 14 is a diagram of a semiconductor substrate with a dualintersecting pass path illustrated thereon;

FIG. 15 is a diagram of a semiconductor substrate with another dualintersecting pass path illustrated thereon;

FIG. 16 is a cross sectional view of a semiconductor substrate beingbackground to remove the damaged layer;

FIG. 17 is a cross sectional view of the semiconductor substrate of FIG.16 following removal of the damaged layer being irradiated using a laseron the second surface a second time to form a damage layer;

FIG. 18 is a cross sectional view of a semiconductor substrate beingbombarded with a plurality of ions from a plasma to form a damage layer;

FIG. 19 is a cross sectional view of a semiconductor substrate beingimplanted with a plurality of ions to form a damage layer;

FIG. 20 is a cross sectional view of a second surface of a semiconductorsubstrate being exposed to a wet etchant with a protective layer on afirst side to form a damage layer;

FIG. 21 is a cross sectional view of a second surface of a semiconductorsubstrate being exposed to a gaseous etchant with a protective layer ona first side to form a damage layer;

FIG. 22 is a cross sectional view of a second surface of a semiconductorsubstrate being rapidly locally cooled to form a damage layer;

FIG. 23 is a cross sectional view of a second surface of a semiconductorsubstrate being rapidly locally heated to form a damage layer; and

FIG. 24 is a cross sectional view of a second surface of a semiconductorsubstrate being rapidly locally cooled while a first surface is beingrapidly locally heated to form a damage layer.

DESCRIPTION

This disclosure, its aspects and implementations, are not limited to thespecific components, assembly procedures or method elements disclosedherein. Many additional components, assembly procedures and/or methodelements known in the art consistent with the intended semiconductorsubstrates and related methods will become apparent for use withparticular implementations from this disclosure. Accordingly, forexample, although particular implementations are disclosed, suchimplementations and implementing components may comprise any shape,size, style, type, model, version, measurement, concentration, material,quantity, method element, step, and/or the like as is known in the artfor such semiconductor substrates and related methods, and implementingcomponents and methods, consistent with the intended operation andmethods.

A wide variety of semiconductor substrate types exist and are used inthe process of manufacturing semiconductor devices. Non-limitingexamples of semiconductor substrates that may be processed using theprinciples disclosed in this document include single crystal silicon,silicon dioxide, glass, silicon-on-insulator, gallium arsenide,sapphire, ruby, silicon carbide, polycrystalline or amorphous forms ofany of the foregoing, and any other substrate type useful forconstructing semiconductor devices. Particular implementations disclosedherein may utilize silicon carbide semiconductor substrates (siliconcarbide substrates). In this document the term “wafer” is also usedalong with “substrate” as a wafer is a common type of substrate, but notas an exclusive term that is used to refer to all semiconductorsubstrate types. The various semiconductor substrate types disclosed inthis document may be, by non-limiting example, round, rounded, square,rectangular, or any other closed shape in various implementations.

Referring to FIG. 1, a cross sectional view of a semiconductor substrate2 is illustrated. As illustrated, the substrate 2 has an originalthickness 4 which was created when it was separated from the boule itwas originally cut or otherwise separated from. Either before processingof the substrate 2 to form semiconductor devices thereon or afterprocessing, the substrate 2 is desired to be thinned to thickness 6.Thinned substrates may provide various desirable characteristics forperformance of the semiconductor devices, such as, by non-limitingexample, lower on resistance, better heat performance, small packageform factor/thickness, etc. Thinning of the substrate may also be drivenby factors that include the inability to get the substrate to separatefrom the boule to produce a substrate below a certain thickness due tosubstrate formation process limitations or wafer processing equipmentlimitations that prevent processing of wafers below a certain thickness.Many reasons may exist that drive the need/interest in thinning thesemiconductor substrate.

The thinning of semiconductor substrates is often done usingbackgrinding, where a backgrinding wheel is applied to the back of thesemiconductor substrate (“back” referring to the side of the substratethat does not have semiconductor devices formed thereon) and operated ina circularly eccentric matter to uniformly remove the material from theentire backside of the wafer. Some backside grinding techniques, such asTaiko grinding, however, do not remove all the material from the entirebackside of the wafer, but leave a ring (Taiko ring” around the thinnedwafer to give it additional support. The ultimate thickness of thesemiconductor substrate following back grinding is determined by variousfactors, including the material properties of the substrate itself.

Various examples of processing methods for thinning semiconductorsubstrates are given in this document using silicon carbide substratesas an example. However, these principles can be applied to many othersemiconductor substrate types, including any disclosed in this document.

Referring to FIG. 1, the substrate 2 has a first surface (side) 8 and asecond surface (side) 10. In various implementations, the first surface8 may be that which has or will have semiconductor devices formedthereon and the second surface 10 may be that which may be referred toas the “back side” of the wafer. As illustrated, a laser beam 12 isirradiating the second surface 10 of the substrate 2. A focal point 14of the laser beam 12 is set so that it is located within thesemiconductor substrate below the second surface 10. The wavelength ofthe laser light used to irradiate the second surface 10 is one for whichthe material of the particular semiconductor substrate is at leastpartially optically transmissive, whether translucent or transparent.The focal point 14 creates a damage area in the substrate 2 at andaround the focal point 14. The degree of damage is determined by manyfactors, including, by non-limiting example, the power of the laserlight, the duration of exposure of the material, the absorption of thematerial of the substrate, the crystallographic orientation of thesubstrate material relative to the direction of the laser light, theatomic structure of the substrate, and any other factor regulating theabsorbance of the light energy and/or transmission of the induced damageor heat into the substrate.

The substrate 2 illustrated in FIG. 1 is a silicon carbide (SiC)substrate, and so the laser light wavelength that may be employed may beany capable of transmitting into the SiC substrate material. Inparticular implementations, the wavelength may be 1064 nm. In variousimplementations, the laser light source may be a Nd:YAG pulsed laser ora YVO4 pulsed laser. In one implementation where a Nd:YAG laser is used,a spot size of 10 microns and an average power of 3.2 W may be usedalong with a repetition frequency of 80 kHz, pulse width of 4 ns,numerical aperture (NA) of the focusing lens of 0.45. In anotherimplementation, a Nd:YAG laser may be used with a repetition frequencyof 400 kHz, average power of 16 W, pulse width of 4 ns, spot diameter of10 microns, and NA of 0.45. In various implementations, the power of thelaser may be varied from about 2 W to about 4.5 W. In otherimplementations, however, the laser power may be less than 2 W orgreater than 4.5 W.

As illustrated, the focal point 14 of the laser light forms a locationof rapid heating and may result in full or partial melting of thematerial at the focal point 14. The point of rapid heating and theresulting stress on the hexagonal single crystal structure of the SiCsubstrate results in cracking of the substrate material along a c-planeof the substrate. Depending on the type of single SiC crystal used tomanufacture the boule, the c-plane may be oriented at an off angle tothe second surface of about 1 degree to about 6 degrees. In variousimplementations, this angle is determined at the time the boule ismanufactured. In particular implementations, the off angle may be about4 degrees.

During operation, the laser is operated in pulsed operation to createnumerous overlapping spots of pulsed light while passing across thesurface of the substrate. As a result, a continuous/semi-continuouslayer/band of modified material is formed within the wafer. In otherimplementations, the laser may be operated in continuous wave moderather than pulsed mode to create the band of modified material. Asillustrated, the stress caused by the focal point 14 causes crackingalong the c-plane in the material of the SiC substrate 2 in one or bothdirections along the c-plane or in both directions. These cracks areillustrated as spreading from the focal point 14 area (where themodified layer/band is located) angled at the off angle in FIG. 1. Invarious implementations, the cracks may be located below the focal point14, above the focal point 14, or spread directly from the focal point14, depending on the characteristics of the laser and the method ofapplication of the laser to the material. In various implementations,the length of the cracks into the substrate is a function of the powerof the laser applied. By non-limiting example, the depth of the focalpoint was set at 500 um into the substrate; where the laser power was3.2 W, the crack propagation from the modified layer/band was about 250um; where the laser power was at 2 W, the crack lengths were about 100um; where the laser power was set at 4.5 W, the crack lengths were about350 um.

As illustrated, the substrate 2 can be indexed below the laser beam 12(or the laser beam 12 can be indexed above the substrate 2, or both canbe indexed together) to produce a plurality of spaced apart locations 16where damage has been induced in the substrate. The width between theplurality of spaced apart locations can be a function of the cracklengths into the material of the wafer, and/or the amount of damagelayer formed as a wafer is initially scanned. By non-limiting example,the width may be set between about the length of the cracks into thewafer to about twice the length of the cracks into the wafer. Insituations where the damage layer is being initially formed duringscanning over the wafer on one side of the wafer, the width between thespaced apart locations can be initially reduced. By non-limitingexample, initially the width may be set to 200 um until the cracks beginto spread from the modified layer, at which point the width (indexamount) may be set to 400 um. The feed speed of the wafer under thelaser (or the laser above the wafer) may be, by non-limiting example,400 mm/second, though slower or faster feed speeds may be used invarious implementations.

While a plurality of spaced apart locations 16 are illustrated, invarious implementations, the laser beam 12 may not be applied in anindexed manner, but may be applied in a continuous or substantiallycontinuous manner to the material of the substrate to create continuousor substantially continuous zones/areas of damage in the substrate.These areas of damage may include corresponding continuous cracking ordiscontinuous cracking of the substrate material. The plurality ofspaced apart locations 16 or the continuous lines/areas affected by thelaser irradiation form a damage layer within the semiconductor substrateafter the laser has completed indexing/passing over the semiconductorsubstrate material.

The objective of forming the damage layer is to break up the structureof the semiconductor substrate material (in the case of SiC, thehexagonal crystalline structure of the substrate). The resulting brokenup structure is then more easily removed by a backgrinding wheel duringbackgrinding operations, as the damaged material of the semiconductorsubstrate no longer can as uniformly resist the abrasion/erosionprocess. In various implementations, the effect of the damage layer maybe observed in various backgrinding parameters, such as, by non-limitingexample, increasing the thinning rate of the semiconductor substrate,increasing a lifetime of a backgrinding wheel, reducing the timerequired to thin the semiconductor substrate to a desired thickness,reducing the wear rate on the backgrinding wheel, or any otherbackgrinding process parameter. The effect on the backgrindingparameters may be observed just while the damage layer is being removedduring the backgrinding process, or may extend beyond the removal of thematerial associated with the damage layer.

In various implementations, the process of irradiating the semiconductorsubstrate with laser light may be repeated one or more times after thedamage layer has been removed after backgrinding, followed by additionalbackgrinding after each laser light irradiation to remove the newlyformed damage layer until the semiconductor substrate has been thinnedto the desired thickness. In other implementations, however, two or morepasses with the laser light may be employed to create the damage layer.In some implementations, two or more passes with focal points atdifferent depths into the semiconductor substrate may be employed toform multiple damage layers or a damage layer of greater thickness intothe material of the semiconductor substrate. These various processingoptions, including multi-pass options, will be discussed in greaterdepth later in this document.

Referring to FIG. 2, another implementation of a SiC substrate 18 isillustrated. Like the substrate in FIG. 1, a plurality of spaced apartlocations 20 have been formed using laser irradiation at a focal point22 indexed across the second surface 24 of the substrate 18. Asillustrated, the damage layer 26 is formed through the effect of theplurality of spaced apart locations 20 and the corresponding cracks 28that extend away from the locations 20 that form lines/layers ofmodified material that extend into the page of the drawing. FIG. 2illustrates how, in various implementations, the formation of the damagelayer 26 can take place on a substrate that has had semiconductordevices formed thereon, illustrated as device layer 30. Depending on thenature of the material in the device layer, in various implementations,laser light passing through the substrate may be reflected back into thesubstrate 18 as illustrated by reflection lines 32 off the device layermaterials.

FIG. 3 illustrates an SiC substrate 34 which is an SiC wafer that hasbeen just separated from an SiC boule using a laser irradiation processlike that disclosed in this document. As illustrated, the effect of thec-plane is to create various striations 36 on the surface of the waferfollowing separation that extend into the material of the SiC wafer atthe off angle (about 4 degrees in this case). The actual shape of thestriations 36 on the wafer in cross section is not as uniform or evenlyspaced as illustrated in FIG. 3, as the shape in cross section in FIG. 3is merely illustrative of the presence of the striations 36 rather thanof their shape. Also, the striations are formed randomly as the materialof the SiC wafer separates from the boule along the shapes of the cracksformed into the material of the wafer, so the regular patternillustrated in FIG. 3 and other figures in this application is merelyintended to show the positioning of the striations 36 on the waferrather than their actual shape. As illustrated in FIG. 3, a firstplurality of spaced apart locations 40 have been formed in the materialof the SiC substrate 34 using a laser with the focal point set at thedepth of the locations 40. A second plurality of spaced apart locations38 is being formed as the laser is indexed across the substrate 34 witha focal point 44 at a second depth less than the first depth into thesubstrate 34. As illustrated, the effect of using the two differentpasses at two different depths is to form a damage layer 42 that isthicker than a damage layer formed in each pass alone.

Referring to FIG. 4, a SiC wafer 46 that has been separated from a SiCboule after previous separation of a previous wafer from the SiC bouleis illustrated. This process (assuming the top surface of the SiC boulewas not ground and polished prior to processing the removal of the SiCwafer 46) results in the wafer 46 having striations 48 on both thesecond surface 50 and first surface 52 of the wafer 46. As illustrated,a plurality of spaced apart locations 54 have been formed by irradiationusing a laser at a focal point 56 into the material of the wafer throughindexing across the second surface 50 of the wafer. The plurality ofspaced apart locations 54 form damage layer 58 throughformation/propagation of cracks 60 from locations 58. FIG. 4 illustrateshow, in particular implementations, a dispersion prevention layer 62 hasbeen applied to the second surface 50 of the wafer 56. This dispersionprevention layer 62 includes a material designed to reduce orsubstantially eliminate dispersion effects of the laser light as itenters the material of the second surface 50 through the striations 48.This material may, in various implementations, be designed to have anindex of refraction for the particular wavelength of laser lightsubstantially similar to the index of refraction of SiC (or any othersemiconductor substrate material being processed). In otherimplementations, the thickness of the dispersion prevention layer abovethe second surface is determined based on the particular wavelength oflaser light being used to reduce and or eliminate the diffraction of thelaser light. In a particular implementation, the where the laser lightwavelength is represented by λ, a formula for the thickness (t) may bet=(λ/4)*(2n+1) where n ranges between 0 and a positive integer.

In various implementations, the material of the dispersion preventionlayer may be, by non-limiting example, polyvinyl alcohol, nonaqueoussoluble polymers, water soluble polymers, water soluble polyester, watersoluble phenol, bisphenol fluorine, poly (penta bromo phenylmethacrylate), poly (penta bromobenzyl methacrylate), iodonapthalene,bromonapthalene, gels, films, optically transmissive polyimides, oils,and any other optically transmissive water or organic based materialcapable of being applied over the wafer. In various implementations ofdispersion prevention layers, nanoparticles including, by non-limitingexample, TiO₂ with a refractive index of 2.1, ZrO₂ with a refractiveindex of 2.3, or any other material with a refractive index similar tothe substrate may be used. In other implementations, however, nodispersion prevention layer 62 may be used.

Referring to FIG. 5, another SiC substrate 64 is illustrated withsimilar striations 66, 68 on the second surface 70 and first surface 72of the substrate 64 like those illustrated in FIG. 4. Since thissubstrate 64 is to be ground and polished on both the second surface 70and first surface 72, laser irradiation has been used to form a firstdamage layer 74 next to the second surface 70 and a second damage layer76 next to the first surface 72. FIG. 5 also illustrates, that, invarious implementations, the laser light 78 can be applied from thefirst surface 70 side of the substrate 64 without flipping the substrateover. FIG. 5 illustrates the case where a single laser light beam isused to form the first damage layer 74 and second damage layer 76serially, or vice versa. In particular implementations, to avoiddiffraction effects resulting from the cracks in the damage layer andthe modified material in the damage layer, the first damage layer 74would be formed first followed by the second damage layer 76 where thelaser is being irradiated from the second surface side. The oppositecourse would be used where the laser is being irradiated from the firstsurface side of the wafer.

FIG. 6 illustrates the case where multiple laser beams 80, 82 are usedto simultaneously form the first damage layer 86 and second damage layer88 by indexing across the wafer on opposite portions of the substrate84. In various implementations, these laser beams 80, 82 are indexingfor at least part of the time spaced apart across a midpoint of thesubstrate 84. FIG. 7 illustrates a substrate 90 where multiple laserbeams 92, 94 are irradiating the substrate simultaneously from thesecond side 96 and first side 98, respectively but at a positionallyaligned portions of the substrate 90, substantially above (or below)each other. The particular alignment of the laser beams in variousimplementations where irradiation occurs from both the second side andfirst side of the substrate may be determined by a wide variety offactors, including, by non-limiting example, laser power, opticalconfiguration, throughput, improvement of damage layer thickness, andany other factor driven by tool configuration, throughput, or processeffectiveness.

FIG. 8 illustrates another SiC substrate 100 where two laser beams 102,104 are being used to simultaneously form a damage layer 106 byirradiating the second side 108 of the substrate 100. As illustrated, inthis implementation, the two laser beams 102, 104 are spaced apartacross a midpoint of the substrate. In various implementations, the twolaser beams 102, 104 may begin on opposite sides of the substrate 100and index towards each other toward the midpoint; in others, they maybegin adjacent to the midpoint, and index away from each other. FIG. 9illustrates another SiC substrate 110 where two laser beams 112, 114 arebeing used to simultaneously form a damage layer 116 in the substrate110 where the two laser beams are spaced closely together. The spacingthe beams 112, 114 may be as close as adjacent locations in theplurality of spaced apart locations 120, or may be any number of spacedapart locations away from each other. In this implementation, and in allother multiple laser beam implementations disclosed in this document,the two or more laser beams used to form the damage layers may have thesame characteristics, or may be different from one another in one ormore of the following respects, by non-limiting example: laser type,laser wave length, spot size, power, pulse energy, pulse width,repetition rate/frequency, indexing speed, dwell time, depth into thesubstrate material, numerical aperture, average power, and any otherdesired laser characteristic. Also, the two or more laser beams may begenerated by the same or different laser devices in variousimplementations.

Referring to FIG. 10, a diagram of a semiconductor substrate 122 isillustrated. The particular semiconductor substrate illustrated here hastwo wafer flats that correspond with an SiC wafer, though the principlesdisclosed herein could be applied to many different substrate types. Asillustrated, a path 124 followed by a laser as it irradiates thesubstrate with laser light is illustrated, with the path 124 indicatinglocations where the light irradiation occurs and a focal point withinthe substrate is formed. In other implementations however, the path 124may illustrate the path of the laser as it travels across the surface ofthe substrate irradiating the wafer in continuous wave rather thanpulsed mode operation. The path 124 illustrated in FIG. 10 is analternating single pass path, where the laser indexes across the waferfirst in the y direction, over in the x direction, and then indexes inthe opposite y direction in various steps across the wafer. In variousimplementations, the spacing of steps in the x direction may the same,as illustrated in FIG. 10. In other implementations, however, thespacing of steps may vary across the wafer, either for an initialperiod, or for the entire distance across the wafer in the x direction,depending on how the damage layer forms. The spacing of the steps may beany disclosed in this document.

FIG. 11 illustrates a semiconductor substrate 128 which has beenprocessed using an intersecting dual pass path 30. In the version of thepath illustrated here, the paths are first irradiated by the laserduring the first pass, and then irradiated again by the laser during thesecond pass. The use of dual pass paths may allow for the enhancement ofthe spreading of the cracking and other damage caused by the laserirradiation by giving the substrate time to cool and/or otherwise adjustthe structure of the substrate between passes. This may, in turn,enhance the thickness or other desired characteristics of the damagelayer formed.

Referring to FIG. 12, a semiconductor substrate 134 which has beenprocessed using a single pass spiral path 136 is illustrated. In variousimplementations, various combinations and arrangements of spiral pathsmay be employed, such as multi-pass paths, and spirals of various shapesand designs (more tightly arranged spirals at the beginning or end ofthe spiral) and various overlapping arrangements of spirals may be used.Also, for spiral (and alternating/intersecting paths), the frequency ofpulses of laser irradiation along the path may be varied along the path(more points at the beginning, middle, or end of the path, or indifferent portions of the path than in other portions).

FIG. 13 illustrates another implementation of a substrate 138 with anintersecting dual pass path 140 where the second pass is angled ratherthan executed at about 90 degrees to the first pass. The angle at whichthe second pass is performed relative to the first pass may bedetermined by various factors, including, by non-limiting example, theorientation of crystallographic planes in the substrates, desiredthroughput rates through the laser process tool, desired crack positionsin the damage layer, and any other process characteristic that affectsthe speed or efficacy of the laser treatment. Note that in FIG. 13 thatsome of the locations along the path of laser irradiation are commonbetween the first pass and the second pass and other locations areunique to one of the passes.

Referring to FIG. 14, a substrate 146 with another implementation of adual intersecting pass path 148 is illustrated. As illustrated, in thisimplementation, all of the locations along the second pass are orientedsubstantially parallel with the locations 152 of the first pass and noneare shared between the two passes. The use of this technique may, invarious implementations, assist with spreading of cracks or other damagewithin the damage layer by allowing the substrate material to react tothe damage of the first pass before the second modified layer of damageis created. FIG. 15 illustrates the case where a substrate 154 isprocessed using a dual intersecting pass path 156 which is executed inthe reverse order from the path 148 illustrated in FIG. 14. In variousimplementations, the dual pass path may be executed in varying ordersfrom substrate to substrate as the damage layer characteristics are notaffected by the order of execution. In other implementations, the orderin which the dual pass path is executed may affect the characteristicsof the damage layer formed, so all substrates have to be executed in thesame order. Where the damage layer characteristics depend on theexecution order of the dual pass path, this may be caused by a widevariety of factors, including, by non-limiting example, thecrystallographic planes of the substrate, the alignment of higher atomicweight atoms in one plane versus and other relative to the direction ofexecution of the dual pass paths, and any other material characteristicsof the substrate and/or the laser light.

Many different single pass, dual pass, and more than two pass paths forprocessing semiconductor substrates may constructed using the principlesdisclosed in this document. Also, many different intersecting, spiral,alternating, alternating+spiral, random, and semi-random paths may beconstructed using the principles disclosed herein. What paths areemployed will depend on many of the different laser and substratematerial factors desired, as well as the desired characteristics of thedamage layer for use in speeding the backgrinding process.

Referring to FIG. 16, an implementation of a substrate 158 with a damagelayer 160 formed thereon is illustrated. By the presence of the angledcracks 162 in the damage layer, this is a SiC substrate. As illustrated,the substrate 158 is being rotated below a grinding wheel which has aplurality of teeth 164 thereon. The life of the grinding wheel isdetermined by how long the teeth 164 remain on the wheel at a usablelength (size). In FIG. 16, the direction of rotation of the grindingwheel against the substrate 158 is illustrated. However, in variousimplementations, the substrate may rotate against the stationarygrinding wheel, the grinding wheel may rotate against a stationarysubstrate, or both the grinding wheel and substrate may rotate relativeto each other. In various implementations, various orbits (circular,eccentric, or otherwise) may be executed by the grinding wheel relativeto the substrate 158 as it contacts the teeth with the substrate.Because the material of the damage layer contacts the teeth first, therate of thinning of the material of the damage layer will be faster thanthe rate of thinning of undamaged material, thus speeding up the overallrate of thinning of the wafer. Also, the useful life of the teeth 164 ofthe grinding wheel may be increased as less wear is induced on the teethdue to the damage layer material being less wearing to remove. Forparticular substrate types, such as silicon carbide, the substrate maybe very resistant to grinding as it is nearly as hard as the material ofthe teeth 164 itself. Accordingly, backgrinding/thinning process of SiCsubstrates takes significant time and results in significant wear on theteeth, increasing the total cost of forming/thinning each SiC wafer.This problem is exacerbated as current substrate forming technologygenerally creates SiC wafers that are thicker than needed because all ofthe fab processing equipment is designed/calibrated to operate usingthicker wafers and/or the wafer separation process is unable to producethinner ones. Because of these factors, the use of a damage layer priorto grinding may reduce the total cycle time and/or increase the life ofeach grinding wheel, thus reducing the cost per wafer to a significantdegree.

As previously discussed, FIG. 17 shows the substrate 158 of FIG. 16following removal of the damage layer through backgrinding beingirradiated again using a laser on the second surface 166 a second timeto form a second damage layer 168. FIG. 17 shows that this process mayalso be repeated additional times to get the wafer down to the desiredthickness as indicated by braces 170. Because the removal rates of thedamage layer 160, 168 material may be much higher than the removal rateof the unaffected bulk material, recursively/repetitively backgrindingbetween each repeated irradiation step may provide a suitable costsaving through reduced cycle time and/or consumable costs for varioussubstrates, particularly SiC substrates.

Various other methods besides irradiation of a surface of asemiconductor substrate may be employed to form a damage layer andreduce backgrinding costs accordingly. Referring to FIG. 18, a crosssectional diagram of a vacuum chamber 172 including a chuck 174 to whicha semiconductor substrate 178 has been coupled. A plasma 180 has beenstruck/formed above the substrate 176 and operating conditions in theplasma 180 have been set to accelerate large quantities of ions 182 fromthe plasma 180 down onto the second surface 178 of the substrate 176.Because the goal of the plasma ions 182 is to damage the second surfaceof the substrate 176 and damage the internal structure of the surface asthey bombard the surface, the operating conditions may be set tomaximize such activity. These operating conditions may go beyond what istypically used during etch processing because normal etch processing hasto avoid damage to underlying substrate materials, which is not an issuehere. A wide variety of gases may be employed to form the plasma andconduct the ion bombardment, including, by non-limiting example, He, Ne,Ar, Kr, Xe, oxygen, fluorine, nitrogen, any combination thereof, and anyother gas type that will tend to not chemically react with the materialof the substrate but pass into it. As the ions penetrate the substratematerial they will interact with and break up the structure of thematerial of the substrate, forming a damage layer more susceptible toremoval as previously described herein. Multiple successive plasmatreatments and backgrinding steps may be employed in variousimplementations to thin the wafer, similar to the multiple laserirradiation steps previously described.

Referring to FIG. 19, an implementation of a semiconductor substrate 184is illustrating being implanted with a beam of ions 186 using an ionimplantation process. The beam of ions in various implementations isvery high energy >200 keV and the current associated with the ion beammay be up to 30 mA or higher to ensure that the ions penetrate as deeplyas possible into the material of the substrate and create as broad adamage layer through breaking up the structure of the material of thesubstrate. A wide variety of implantation processes may be employed andmultiple implantation/backgrinding steps could be employed in variousimplementations. Examples of the ions that could be used forimplantation may include, by non-limiting example, nitrogen, boron,argon, and any other ion type not as likely to chemically react with thematerial of the particular semiconductor substrate being implanted. Theion type may also be chosen based on the ion's ability to break up theatomic structure of the substrate being implanted. In variousimplementations, implanting using a proton beam or beam of othersubatomic particles may be used. Sets of implantation parameters thatachieve a substantially square (BOX) profile of penetration of the ionsor particles may be utilized in various implementations to maximize thedamage layer's depth/uniformity.

FIG. 20 illustrates a semiconductor substrate 188 immersed in a bath 190filled with a liquid 192. One side of the substrate 188 is covered witha protective layer 194 (film, sheet, deposited film, spray or spun oncoating, etc.) to prevent it from contacting the liquid 192. The liquid192 may include any of a wide variety of chemical compounds designed tochemically interact with the structure of the particular material of thesubstrate 188 such as, by non-limiting example, acids, bases, electronreceptors, electron donors, compounds that react selectively withcarbon, compounds that react selectively with silicon, compounds thatreact selectively with a specific crystallographic plane of the materialof the semiconductor substrate 188, hydrofluoric acid, phosphoric acid,nitric acid, acetic acid, and any other chemical capable of damaging thesecond surface 196 of the substrate 188. In these variousimplementations the liquid acts to create a damage layer into thematerial of the substrate 188 which can then be more easily backgroundfrom the substrate 188 as previously discussed. Multipleetching/backgrinding passes on the wafer may be used in the thinningprocess in various implementations.

Referring to FIG. 21, a semiconductor substrate 198 is illustratedsupported in a furnace 202 with a protective layer 204 coupled to a sideof the wafer to prevent it from contacting the gas in the furnace 202. Agaseous etchant 204 is introduced into the furnace 202 and designed toreact with the second surface 206 of the substrate 198 to form a damagelayer into the substrate. A wide variety of gaseous etchants may beemployed in various implementations, such as, by non-limiting example,hydrochloric acid, fuming nitric acid, sulfuric acid, hydrofluoric acid,strong bases and any other gaseous etchant may be utilized in variousimplementations. Both vertical and horizontal diffusion furnaces may beutilized to process many substrates at a time, or single substratechambers may be employed in various implementations. Also, as previouslydiscussed, multiple etching/backgrinding passes on the substrate may beemployed as the substrate is thinned to a desired thickness.

Referring to FIG. 22, an implementation of a substrate 208 coupled witha rapid local cooling apparatus 210 is illustrated. As illustrated, therapid thermal cooling apparatus 210 is designed to rapidly cool aportion of the substrate 208 a desired depth 212 into the wafer. Basedon the coefficient of thermal expansion of the material of substrate208, the result of the cooling may cause high stressing of the materialof the substrate 208 to the desired depth. The high stress conditionover a short period of time causes breakup and damage to the material ofthe substrate to the desired depth 212, forming a damage layer. Thedamage layer can then be removed after the substrate 208 is decoupledfrom the rapid local cooling apparatus 210 through backgrinding. As withthe other methods for forming damage layers, multiple cooling andbackgrinding cycles may be employed in the process of thinning thesubstrate 208. The rapid local cooling apparatus 210 may take manyvarious forms and employ various structures. For example, in oneimplementation, liquid nitrogen, ammonia, or another liquefied coolantmay be passed through into a distribution plate 214 coupled to thesubstrate 208. In various implementations, the distribution plate may bepre-cooled before contacting the substrate 208, or it may be cooledafter being coupled to the substrate. In various implementations, anintermediate material/structure may be used to couple the substrate 208to the distribution/cooling plate 214, including, by non-limitingexample, a thermal grease, a water soluble gel, an adhesive, a magneticcoupler, a clamp, and any other system or method of holding thesubstrate to the distribution plate.

FIG. 23 illustrates a semiconductor substrate 216 being rapidly locallyheated by a heat source [represented by a light source 218 in FIG. 23,but the heat source could also be a thermally conductive/convective heatsource (electrical, combustive, plasma, etc.) in variousimplementations]. As illustrated, the rapid local heating of thesubstrate 216 results in damage to the structure of the substrate downto a certain level within the substrate due to high stressing of thematerial based on coefficient of thermal expansion effects. This forms adamage layer 220 in the substrate from the second side 222 of thesubstrate. As previously discussed, following backgrinding of thematerial of the damage layer 220, additional rapid localheating/backgrinding steps may be utilized in the process of thinningthe substrate 216 to the desired thickness. As with the other damageinducing processes discussed in this document, the number ofbackgrinding steps depends on the depth of the damage layer formedthrough the particular damaging technique employed. A wide variety ofequipment types could be used to conduct the rapid thermal heating,including, by non-limiting example, rapid thermal annealing (RTA)equipment using light irradiation, short dwell times on preheated ovenplates, ovens applying superheated gas jets to the second surface of thesubstrate, and any other system designed to rapidly heat the second sideof the substrate in a very short period of time.

Referring to FIG. 24, an implementation of a system that rapidly locallyheats the first side 230 of a semiconductor substrate 224 while rapidlylocally cooling the second side 232 of the substrate 224 simultaneouslyis illustrated. The system used to rapidly locally cool may be anydisclosed in this document, as may the system for rapidly locallyheating the substrate. Systems that employ rapid local heating andcooling simultaneously may form damage layers 234, 236 adjacent to thesecond side 232 and first side 230 of the substrate, respectively. Thismay allow the wafer to be thinned more rapidly simply by being able tohave the backgrinding tool remove the damage layer from both sides ofthe wafer in two separate backgrinding steps, allowing for potentiallytwice as much damage layer material to be removed in each thinningcycle. However, in other implementations, the use of this technique maybe designed, depending on the coefficient of thermal expansion of thematerial of the semiconductor substrate, to create a broader damagelayer 238 adjacent to either the first surface 230 or second surface 232of the substrate 224. This broader damage layer may result from, bynon-limiting example, the position of the peak temperature within thematerial of the substrate, the position of the peak high or peak lowtemperature of the transient thermal gradient created within thematerial of the substrate, the duration of the temperature swing fromlow temperature to high temperature or from high temperature to lowtemperature, or any other transient thermal phenomenon within thematerial of the substrate.

While the use of simultaneous rapid local heating and local cooling hasbeen described in reference to FIG. 24, in other implementations, eitherthe heating or the cooling may not be done simultaneous with the rapidcooling or rapid heating. Instead, the either the heating or coolingsystem may be used to bring the substrate 224 to a steady state elevatedor lowered temperature relative to ambient temperature and then thesubstrate 224 is subjected to a rapid local cooling or rapid localheating process, respectively. The use of preheating or precooling thesubstrate may assist in widening the width of the damage layer orincreasing the damage in the damage layer on the side of the wafer beingrapidly thermally processed by increasing the local temperature gradientbetween the rapid thermal heating or cooling process. A wide variety ofpotential variations may be constructed using the principles disclosedin this document.

Various methods of forming damage layers and techniques of forming thesame have been disclosed in this document. While these have beendescribed separately, in various semiconductor substrate thinningmethods and systems, any combination of the various methods may beutilized together in combination in various implementations. Bynon-limiting example, a damage layer(s) may first be formed using any ofthe laser irradiation methods disclosed in this document and may then befollowed by further damage layer formation using ion bombardment, rapidheating/cooling, or any combination thereof. A wide variety of variousdamage layer formation methods using various combinations of thedifferent damage layer formation methods disclosed in this document maybe constructed using the principles disclosed herein.

In places where the description above refers to particularimplementations of semiconductor substrate thinning methods and systemsand implementing components, sub-components, methods and sub-methods, itshould be readily apparent that a number of modifications may be madewithout departing from the spirit thereof and that theseimplementations, implementing components, sub-components, methods andsub-methods may be applied to other semiconductor substrate thinningmethods and systems.

What is claimed is:
 1. A method of thinning a semiconductor substratecomprising: providing a semiconductor substrate having a first surfaceand a second surface opposing the first surface; inducing damage into aportion of the semiconductor substrate adjacent to the second surfaceforming a damage layer; and backgrinding the second surface of thesemiconductor substrate.
 2. The method of claim 1, wherein a lifetime ofa backgrinding wheel is increased through the presence of the damagelayer.
 3. The method of claim 1, further comprising one of: afterbackgrinding the second surface of the semiconductor substrate, inducingdamage into a portion of the semiconductor substrate adjacent to thesecond surface forming a second damage layer and backgrinding the secondsurface of the semiconductor substrate to remove at least the seconddamage layer; after inducing damage into the portion of thesemiconductor substrate, again inducing damage into the portion of thesemiconductor substrate adjacent to the second surface beforebackgrinding the second surface of the semiconductor substrate.
 4. Themethod of claim 1, wherein a thinning rate is increased while abackgrinding wheel grinds the damage layer.
 5. The method of claim 1,wherein inducing damage into the portion of the semiconductor substratefurther comprises irradiating the second surface with a laser beam at afocal point within the semiconductor substrate at a plurality of spacedapart locations along the second surface to form the damage layer. 6.The method of claim 5, wherein the method further comprises forming oneor more cracks into the semiconductor substrate surrounding each of theplurality of spaced apart locations.
 7. The method of claim 5, whereinthe semiconductor substrate is silicon carbide.
 8. The method of claim5, wherein irradiating the second surface with the laser beam at thefocal point within the semiconductor substrate at the plurality ofspaced apart locations further comprises irradiating the plurality ofspaced apart locations using a predefined path.
 9. The method of claim8, wherein the predefined path is one of an alternating single passpath, an intersecting single pass path, a spiral single pass path, analternating dual pass path, a dual intersecting pass path, a spiral dualpass path, a random single pass path, a random dual pass path, a singlepass path, a two or more pass path, an intersecting dual pass path, anoverlapping single pass path, an overlapping dual pass path, and anycombination thereof.
 10. The method of claim 1, wherein inducing damageinto the portion of the semiconductor substrate further comprisesbombarding the second surface with a plurality of ions from a plasmaadjacent to the second surface to form the damage layer.
 11. The methodof claim 1, wherein inducing damage into the portion of thesemiconductor substrate further comprises implanting the second surfacewith a plurality of ions to form the damage layer.
 12. The method ofclaim 1, wherein inducing damage into the portion of the semiconductorsubstrate further comprises exposing the second surface to an etchant toform the damage layer.
 13. The method of claim 1, wherein inducingdamage into the portion of the semiconductor substrate further compriseslocally rapidly cooling the second surface to form the damage layer. 14.The method of claim 1, wherein inducing damage into the portion of thesemiconductor substrate further comprises locally rapidly heating thesecond surface to form the damage layer.
 15. The method of claim 13,further comprising one of: locally rapidly heating the first surfacewhile locally rapidly cooling the second surface to form the damagelayer; and locally rapidly heating the second surface while locallyrapidly cooling the first surface to form the damage layer.
 16. A methodof preparing a semiconductor substrate for thinning, the methodcomprising: providing a semiconductor substrate having a first surfaceand a second surface opposing the first surface; and forming a damagelayer in a portion of the semiconductor substrate adjacent to the secondsurface, the damage layer configured to increase a thinning rate when abackgrinding wheel is grinding the damage layer.
 17. The method of claim16, wherein forming the damage layer in the portion of the semiconductorsubstrate adjacent to the second surface further comprises irradiatingthe second surface with a laser beam at a focal point within thesemiconductor substrate at a plurality of spaced apart locations alongthe second surface to form the damage layer.
 18. The method of claim 17,wherein the substrate is silicon carbide.
 19. The method of claim 17,wherein the method further comprises forming one or more cracks into thesemiconductor substrate surrounding each of the plurality of spacedapart locations.
 20. The method of claim 16, wherein forming the damagelayer in the portion of the semiconductor substrate adjacent to thesecond surface further comprises one of: bombarding the second surfacewith a plurality of ions from a plasma adjacent to the second surface toform the damage layer; implanting the second surface with a plurality ofions to form the damage layer; exposing the second surface to an etchantto form the damage layer; locally rapidly cooling the second surface toform the damage layer; locally rapidly heating the second surface toform the damage layer; locally rapidly heating the first surface whilelocally rapidly cooling the second surface to form the damage layer;locally rapidly heating the second surface while locally rapidly coolingthe first surface to form the damage layer; and any combination thereof.